[experiments][models][experiment] Fix exception declaration in except block

beat/web/experiments/models/experiment.py

@@ -196,8 +196,8 @@ class ExperimentManager(ContributionManager):
         # Save the experiment (will run the validation)
         try:
             experiment.save()
-        except SyntaxError, e:
-            return (None, None, e.message)
+        except SyntaxError as e:
+            return (None, None, str(e))
         except Exception:
             import traceback
             return (None, None, traceback.format_exc())